So, while I was exploring Lumiose (mostly just walking like crazy to hatch an egg... which still hasn't hatched yet) I randomly encountered a

Code:

ghost girl in an office building who floated by me and said "you're not the one."

I'm curious if that's a one-off or if that's a hint to a further sidequest.

I am betting that is a location where a specific event Pokémon triggers some event (or a Mystery Gift key item that triggers a legendary). So assuming it is related to a ghost that would mean Giratina or maybe Hoopa.